In diagnostics utilizing nuclear magnetic resonance, magnetic field strength corresponds to a place to be diagnosed. Accordingly, the accuracy required for the strength of the magnetic field generated by a magnet system is on the order of one millionth of the magnetic field strength. By the way, the magnetic fields relating to a nuclear magnetic resonance imaging apparatus (hereinafter referred to as “MRI apparatus”) is broadly divided into three types as follows:
(1) Static magnetic field that is temporally stationary and spatially constant, the strength of which is typically 0.1 or several teslas or more. This static magnetic field varies only within the range of a few ppm in a space in which imaging is performed (typically a spherical or ellipsoidal space with a diameter of 30 or 40 cm or so, herein referred to as “imaging field”).
(2) A gradient magnetic field that is varied with a time constant of about one second or less and is spatially tilted.
(3) A magnetic field generated by a high-frequency electromagnetic wave with a frequency corresponding to nuclear magnetic resonance (a several MHz or higher).
The static magnetic field spatially constant and temporally stationary of (1) described above is typically generated by a permanent magnet or a coil in which current flows. This static magnetic field can also be more homogeneous by magnetizing magnetic substances appropriately disposed.
The gradient magnetic field of (2) described above is generated by a coil in which temporally varying current flows (hereinafter referred to as “gradient magnetic field coil”). The frequency of the applied nuclear magnetic resonance of (3) described above is associated with the position of the nuclear magnetic resonance by applying gradient magnetic field. This gradient magnetic field cannot be generated by only a coil having a simple circular pattern of conductor.
A coil that generates the gradient magnetic field of (2) described above has a shape of a saddle and a complicated pattern of wiring passing between parallel surfaces (i.e., a three-dimensional wiring), as described on Patent Document 1. In order to determine such a complicated pattern for a target magnetic field, a method for calculating a coil pattern that generates the target magnetic field. Also, what coil pattern can be designed depends on what method is used for calculating the coil pattern.
In the example of conventional art described above, the surface on which a coil pattern is arranged is represented by a combination of square finite surface elements F, as shown in FIG. 14(a). As shown by arrows indicating rotation directions in this figure, currents are considered that circulate within the respective finite surface elements, and the magnitude of each current is determined so that the target magnetic field can be reproduced. The current flowing in each side is determined from the difference of currents circulating adjacent finite surface elements F. Consequently, the whole current flow can be represented by lines as schematically shown in FIG. 14(b). However, this method has problems as follows:
(2-1) Net Current that Flows in a Hollow Ring-Shaped Surface Cannot be Represented.
Although current that flows as eddy current can be represented, current that flows in one direction as a whole cannot be represented. For example, in a cylindrical system as shown in FIG. 15, net current that flows in a circling direction cannot be represented using this conventional method. This is because the conventional art example assumes that no net current exists in currents circulating within the respective square elements. According to this assumption, in order to simulate current in the circling direction, current in the reverse direction needs to be added somewhere. Or a surface with no hole needs to be a calculation surface.
(2-2) A Fabricatable Coil Pattern Cannot be Determined.
In the current pattern of the conventional art example, the portion of the current pattern flowing between a top surface F1 and a bottom surface F2 in FIG. 14(b) needs conductive connections, in which the coil pattern would be desirably linear, but the conventional art example only allows a curved coil pattern. Also, by the same reason as (2-1), a coil pattern in which the conductive connections are disposed centered to a certain area cannot be designed.
(2-3) Calculation with Incoming/Outgoing Currents Considered is Impossible.
Since only circulating currents are assumed, an incoming point at which current flows into the surface and an outgoing point at which current flows out of the surface, which may exist in an actual pattern, cannot be represented. This is because, in the gradient magnetic field coil (GC) including the combination of two surfaces shown in FIG. 14(b), both the surfaces need to be calculated at one time to determine the coil pattern. This makes it difficult to support at the same time the shielding capability with a required accuracy of an error magnetic field of 0.1 gauss or less and the gradient magnetic field of imaging field allowing an error magnetic field of several gausses, resulting in a useless pattern.
(2-4) Difference in Magnetic Field Accuracy Between Regions Cannot be Reflected in the Calculation.
Difference in required magnetic field performance between the top surface F1 and the bottom surface F2 cannot be reflected in the calculation, resulting in the problem as described above.
Due to the limitation of this method, in the conventional art example, the conductive connections between the top surface F1 and the bottom surface F2 are curved and swollen as shown in FIG. 14(b). This makes the fabrication difficult. Also, when the whole system is not cylindrical but, for example, in the shape in which a hole exists in a curved surface, such as a pot-like shape, the pattern is limited (in the case of (2-1), net current cannot be represented). This makes it difficult to achieve target performance and to fabricate the coil.    Patent Document 1: U.S. Pat. No. 5,309,107